The present invention relates to a method and apparatus for modulating phase locked loops, and more particularly to a method and apparatus of this type in which the frequency range of modulation of the phase locked loop is extended substantially over that of prior art modulation techniques.
In general, a phase locked loop includes a voltage controlled oscillator and a source of a reference signal. A phase detector compares a component of the reference signal, i.e., phase, with the same component of the output signal from the voltage controlled oscillator. The phase detector produces an output signal related to the difference in the compared components of the two signals. This output signal is processed in a loop filter, for example a lowpass filter, and the processed signal is applied to an input terminal of the voltage controlled oscillator to control the frequency of the output signal from the oscillator. In the past, modulation of the phase locked loop, i.e., modulation of the output signal from the voltage controlled oscillator, was accomplished by applying a modulation signal to one or both of two points in the phase locked loop.
In a first prior art technique for modulating the phase locked loop, the reference signal is modulated. The modulation response of the phase locked loop for this type of modulation has a transfer function H(.omega.) which is relatively flat, i.e., constant, for low frequencies up to approximately the natural or resonant frequency of the phase locked loop. However, the transfer function decreases rapidly at approximately the natural frequency of the loop and it is therefore generally impractical to use this method of modulation for frequencies greater than the natural frequency of the loop, in view of the rapidly decreasing loop response at these frequencies.
In a second method of modulating phase locked loops, the modulation signal is summed with the control input signal to the voltage controlled oscillator. The modulation response of the loop for this type of modulation can be defined as 1-H(.omega.), which is relatively flat for frequencies greater than the natural frequency of the loop. However, this transfer function decreases rapidly for frequencies less than the natural frequency. Previous attempts to extend the low frequency response range using this modulation technique have employed pre-emphasis circuits which amplify the modulating signal to compensate for loop attentuation of the modulation. If the phase locked loop utilizes complex loop filters to provide large amounts of reference frequency attenuation, the required modulation compensation circuit is very complex and difficult to align with the loop response to provide a flat net modulation response.
A proposed solution for extending the frequency range of the modulation of a phase locked loop is disclosed in U.S. Pat. No. 4,052,672, issued to Enderby et al. In the phase locked loop disclosed in that patent, a modulation signal is directly summed with the input signal to the voltage controlled oscillator, and thus provides substantially linear high frequency modulation response in accordance with the previously desribed prior art technique. In addition, the modulation signal is integrated and the integrated signal is summed with the output signal from the phase detector of the loop. This integration is performed to compensate for the fact that the phase locked loop has a low frequency transfer function which approximates that of a differentiator. The integration of the modulation signal cancels out the differentiation response of the phase locked loop and provides substantially linear modulation response at low frequencies. However, the modulation technique disclosed in the Enderby et al. patent does not provide for linear modulation response in the mid-frequency range. This problem is due in large part to the parasitic or non-linear frequency and phase response effects of the phase detector and other loop circuitry located ahead of the point at which the integrated modulation signal is added to the loop signal. In other words, the proposed solution of the Enderby patent is applicable only to phase locked loops utilizing ideal phase detectors, i.e., those providing a transfer function K/j.omega. for all frequencies.
One common application of a modulated phase locked loop is as a frequency synthesizer. The industry trend today is towards frequency synthesizers having relatively low sideband noise levels. The lowering of the noise level in the output signal of the frequency synthesizer by increasing the phase detector gain constant is frequently accompanied by an increase in the parasitic effects of the phase detector and other loop components. The proposed solution of the Enderby et al patent is not suitable for use in relatively low noise frequency synthesizers having increased phase detector and loop filter parasitic effects.
In addition, the proposed solution of the Enderby et al patent is further limited in that the integrated modulation signal can only be summed with the loop signal at a junction between the phase detector and the loop filter. The signal cannot be added to other points in the loop since the integration technique alone is not capable of accounting for modulation response of the loop if a signal is added at other points. This limitation presents a drawback in that undesirable circuit partitioning may result from arranging the phase locked loop to accommodate the addition of the integrated modulation signal at this point. For example, if the phase locked loop is incorporated in an integrated circuit, it may be necessary to add a number of extra pins to the integrated circuit to allow the signal to be added at the desired junction. Among other factors, the need to provide additional pins results in increased manufacturing costs.